Steel material of Fe-C alloy is one of the most general metal materials. Particularly, steel materials containing some elements are referred to as special steel, and widely used as raw materials for structural members, tools, and jigs.
Elements contained in the special steels include Al, B, Co, Cr, Mn, Mo, N, Ni, Pb, S, V, Ti, Ta, W, and Zr. These elements improve the characteristics of steel materials by being contained in certain proportions. For example, a boron steel containing B in the range from 40 to 70 ppm by weight (hereinafter, the unit of “ppm” refers to “ppm by weight” unless otherwise specified) exhibits better mechanical strength, hardness, and toughness than general steel materials. A steel containing Pb is known as a free cutting steel that can highly easily be cut off.
Those elements in steel materials are present in different states. Almost all elements are present as a solid solution or compound with ferrite (a solid solution with α-Fe and C) of the steel material, or as a solid solution or compound with cementite (Fe3C). Some elements may be present as nonmetal compounds such as oxides or sulfides or intermetallic compounds. In the Pb free cutting steel, Pb is present by itself in the steel material without being bonded to other elements.
While a steel material is being rolled, forged or otherwise plastically formed into a desired shape, it is customary to treat the surface thereof by hardening, carburizing, nitriding, etc. In the hardening process, the surface of a steel material is heated to produce an austenite (a solid solution with γ-Fe and C), and then rapidly cooled to produce a martensite. In the carburizing and nitriding processes, after a steel material is heated, C or N is introduced into the steel material from its surface. These surface treatment processes result in a case-hardened steel material.
A boron steel tends to crack while being quenched, and any boron steel workpieces with cracks cannot be used as products. Stated otherwise, when a boron steel is quenched, the yield is lowered. The reason why a boron steel tends to crack is that a trace of Fe, C, Si, Ni, Mo, or the like which is present as a separate impurity in the steel material reacts with B, generating a brittle material such as FeB, Fe2B, Fe5SiB2, Ni4B3, MoFeB4, MO2FeB2, B4C, or the like which is precipitated and localized in the crystal grain boundary of the steel material. The brittle material thus present is liable to suffer large thermal stresses in the steel material when the steel material is quenched.
While a boron steel has good mechanical strength, hardness, and toughness at its surface, these properties are not good enough in the inner structure of the steel because it is difficult to introduce or diffuse boron deeply into the steel material. Boron quickly reacts with the above separate impurity when the steel material is boronized.
When a steel material is carburized or nitrided, C or N is diffused into the steel material usually by a distance of about 0.1 mm or slightly over 0.25 mm at maximum from its surface. Therefore, while the carburizing or nitriding process is effective to harden the surface of the steel material, it fails to harden the inner structure of the steel material beyond the distance of 0.3 mm from the surface. In addition, the toughness of the carburized or nitrided steel material is lower than before it is carburized or nitrided.
Japanese laid-open patent publication No. 53-142933 proposes another surface treatment process of nitriding a steel material and then boronizing the steel material. According to the proposed surface treatment process, a temperature of heating the steel material in the boronizing process can be lower than a temperature in a boronizing process where the steel material would not be nitrided. Hence, the steel material can be formed into less strained products.
However, as described in the above publication, an Fe—B—N compound is generated only in the surface of the steel material according to the proposed surface treatment process. Since B or N does not enter deeply into the steel material, it is difficult for the process to improve the properties of the steel material in its inner structure.